Electroluminescent displays are well known in the art. Thick dielectric electroluminescent displays have been developed that provide a very high luminance as described, for example, in Applicant's U.S. Pat. No. 5,432,015, the subject matter of which is incorporated herein by reference.
Improvement to thick dielectric electroluminescent displays relate to the use of a blue light-emitting electroluminescent phosphor and a photoluminescent colour conversion layer to generate red and green light, which is described in Applicant's co-pending U.S. patent application Ser. No. 10/661,728, the subject matter of which is incorporated herein by reference. Further developments of photoluminescent colour conversion materials used in the colour conversion layer have been obtained to improve the luminance and operating stability of the display, which is described in Applicant's co-pending U.S. Provisional Patent Application 60/571,941, the subject matter of which is incorporated herein by reference.
Provided that substantially all of the blue light emitted from the blue light-emitting electroluminescent phosphor is absorbed by the colour conversion layer, the emission colour of the colour conversion layer is dependent upon the photoluminescent colour conversion material and not the light from the blue-emitting electroluminescent phosphor used to excite the colour conversion materials. With careful selection of the colour conversion materials, the emission spectra for red and green sub-pixels can approximate that required to meet commercial television application standards such as the National Television System Committee (NTSC) or European Broadcasting Union (EBU) colour gamut standards. Nevertheless, some colour correction is required to fully meet these standards. For example, the contrast and colour balance of a display can be adversely affected by the reflection of ambient light from the display surface or from the interfaces between different layers in the display structure. Typically, achievement of adequate contrast requires the use of a separate contrast enhancement filter and a separate colour-correcting optical filter. The combination of filters is designed to minimize the reflectance of ambient light while minimizing absorption of the light emitted from the display. The filters may each be deposited on the front viewing surface of the display, which in the case of an electroluminescent display incorporating a colour conversion layer, may be on the colour conversion layer itself. Alternatively, the filters can be deposited on a separate sheet of glass which is then aligned and affixed to the viewing side of the display. The separate sheet of glass may also serve to protect the active portion of the display from degradation due to atmospheric contaminants such as oxygen or water.
Typically, each of the contrast enhancement filters, each of the separate colour-correcting optical filters, and each of the colour conversion layers have to be patterned separately since a particular filter layer differentially absorbs light from red, green and blue sub-pixels. Different filter layer materials are required for each. If the filters are formed on a separate sheet of glass, the filter patterns must be accurately aligned with the underlying sub-pixel structure, which has a similarly patterned colour conversion layer.
It is therefore desirable to provide a colour electroluminescent display in a more cost effective and operationally effective manner that can be fabricated more easily, providing a desired colour correction.